1. Field of the Invention
The present invention relates to a microwave tunable filter, and more particularly, to a microwave tunable filter within a millimeter band using microelectromechanical systems (hereinafter, referred to as xe2x80x98MEMSxe2x80x99).
2. Discussion of Related Art
Referring to FIGS. 1 and 2, the construction and operation of conventional microwave tunable filters are firstly described.
FIG. 1 is an exemplary view illustrating the construction of the conventional microwave frequency multiplexing system using multiple channel filters and switches. As shown, filters 1 to 3 corresponding to the number of the multiple channels are connected in parallel to each other, and then only a desired channel signal is transmitted and processed by the operation of switches 4 and 5.
In this case, since the number of filters corresponds to the number of multiple channels, the size of the frequency multiplexing system should be bulk and accordingly the cost of production should be high. In addition, upon switching of the desired filter, the unnecessary power consumption caused due to each switch can not be avoided.
To solve this problem, there is provided another conventional microwave tunable filter using unit resonant cells, as shown in FIG. 2.
As shown, a single unit resonant cell 12 is comprised of an inductor 6, a capacitor 7, a transmission line 8 and a varactor 9.
The varactor 9, which is a kind of variable capacitance diodes, is used in a microwave circuit in such a manner that the capacitance of varactor 9 was changed by the application of a reverse voltage to a pn junction.
Under the above construction, the unit resonant cells 12 to 14 are connected by means of an appropriate coupling to embody the microwave tunable filter.
The transmission line 8 can be formed by a microstripline or a coplanar waveguide and so on.
The center frequencies of the unit resonant cells 12 to 14 are converted in accordance with the variation of the capacitance of each varactor 9 to 11 which is made by the application of the bias voltage from the outside.
If the capacitance of the each varactor 9 to 11 is varied, the center frequencies of the unit resonant cells 12 to 14 are converted, which results in the conversion of the center frequency of the microwave tunable filter.
Instead of using the varactors 9 to 11, transistors or yttrium iron garnets can be used and in this case, of course, the basic construction of the microwave tunable filter is the same as FIG. 2.
It should be, however, noted that the conventional microwave tunable filters as shown in FIGS. 1 and 2 have some problems to be solved as follows:
firstly, in case of using the varactor, since the varactor has a low Q value, the loss of filter is increased due to the low Q value of the varactor in high frequency region; and
secondly, the operation of varactor consumes the DC power and thereby, a high-frequency characteristic is deteriorated by the thermal degradation.
Accordingly, the present invention is directed to a microwave tunable filter that substantially obviates one or more of the problems due to limitations and disadvantages of the related arts.
An object of the invention is to provide a microwave tunable filter which can have the following advantages: a) the integration of MEMS tunable filter and MMIC; b) the very low signal transmission loss and low dispersion; and c) the drastic variation and linear characteristic of frequency by means of MEMS capacitor and an external control signal.
According to an aspect of the present invention, there is provided a microwave tunable filter using MEMS capacitors comprising a plurality of unit resonant cells, each unit resonant cell being formed by various serial and parallel combination of an inductor, a capacitor, a transmission line, and a variable MEMS capacitor, whereby capacitance variation of the MEMS capacitor in each of the unit resonant cell converts a resonant frequency of each of the unit resonant cell to thereby convert a center frequency of the filter.
In the embodiment of the present invention, a bias voltage, which varies the capacitance of the variable MEMS capacitor, is applied between the variable capacitor and ground via a bias voltage source and a high frequency choke for blocking a high frequency signal.
According to another aspect of the present invention, a microwave tunable filter using an MEMS capacitors comprising: a plurality of unit resonant cells each having variable MEMS capacitors and coupled properly to the unit resonant cell adjacent thereto for obtaining a microwave band pass filter characteristic; and a microwave choke portion having both ends connected correspondingly with a bias voltage source and each of the unit resonant cells, for performing the appliance of a low frequency voltage between the variable MEMS capacitors of the unit resonant cell and ground and for blocking the application of a microwave signal inputted from an input terminal of the filter to the bias voltage source.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed.